1. Field of Invention
The present invention relates to a motion transmitting device, particularly to an electromagnetic device for transmitting rotary motion to a sealed chamber, particularly, a vacuum chamber. The invention also relates to a method of heat treatment, particularly baking the interior of a sealed vacuum chamber.
2. Description of Prior Art
It is often desirable or necessary to rotate objects within a sealed chamber which has a pressure differential with the outside. For instance, when a test specimen, such as a semiconductor wafer, is mounted on a shaft in a chamber, it is often necessary to change the position of the specimen. For this purpose, rotary motion feedthrough mechanisms have been known and used in the past. Specifically, such a mechanism comprises a shaft which extends from inside the sealed chamber to the outside via a rotary feedthrough connection so that the shaft can be rotated from outside of the chamber. The feedthrough connection is an airtight device which retains the pressure differential between the chamber and the outside. Such an arrangement is especially useful where it would be impractical to open the chamber merely to rotate the object.
In those cases where it is necessary to reposition a specimen in a sealed vacuum chamber via a stepper motor under program control, the above-mentioned feedthrough mechanism is used as an intermediate sealed mechanical drive device for transmitting motion from the external stepper motor to a specimen holder located inside the chamber. The provision of the feedthrough mechanism increases the weight of the system as a whole, and makes it more expensive to manufacture. One such feedthrough mechanism is described in page 96, the Catalog of Huntington Mechanical Laboratories, Inc., 1990, p. 96, Mountain View, Calif.
As is known in the art, after assembling and prior to use, a vacuum chamber is normally subjected to a heat treatment procedure known as baking. Baking consists of heating the interior of the chamber to a temperature of up to 450.degree. C., e.g., by infrared lamps located inside the chamber, while the chamber is maintained under vacuum. Baking is necessary for cleaning the interior of the chamber from absorbed gases and other impurities, and especially for evaporating and removing moisture which can adhere to the inner walls of the chamber and which cannot be easily removed by vacuum. Upon completion of baking, the infrared lamps are removed from the chamber, and the chamber is evacuated and sealed.
In case the vacuum chamber is equipped with a motor-driven feedthrough device, baking requires that the electric motor be removed, since, when the temperature exceeds the Curie point, the magnets in the motor may lose their magnetism. Therefore additional time is required for removing the electric motor. (As the feedthrough mechanism itself is sealed by a bellows, it remains hermetically sealed after removal of the motor.) Upon completion of baking, additional time is required for reinstalling the motor in place.
Another essential drawback of feedthrough devices with bellows-type seals is a possibility of occasional rupture or failure of the bellows, e.g., due to fatigue. In case the chamber contains a poisonous material, such as gaseous cyanide used in a technological process inside the chamber, the failure of the bellows will allow the gas to leak to the outside, presenting a serious health problem for personnel.